Pure titanium exists in the alpha crystalline form at room temperature but transforms to the beta crystalline form at temperatures greater than 1621.degree. F. Various alloying elements increase the stability of the beta phase at lower temperatures. Certain known titanium alloys contain sufficient amounts of the beta phase stabilizers so that they are largely beta phase under most temperature conditions and are referred to as beta titanium alloys. However this class of such alloys are not 100% beta phase but include some amounts of the alpha phase which acts as a strengthening phase but which disappears with increasing temperature, leading to a pronounced in strength at elevated temperatures. The subject of these prior "beta" titanium alloys is discussed in "The Beta Titanium Alloys" by F. H. Froes et al, Journal of Metals 1985 pp. 28-37. I know of no commercial titanium alloys which are true 100% beta phase alloys under all conditions of temperature.
Titanium alloys posses an ideal combination of strength and low density for many aerospace applications including gas turbine engines and particularly gas turbine engine compressor blades, vanes and related hardware. However, titanium is a highly reactive metal and can undergo sustained combustion under conditions encountered in gas turbine engine compressors. In such compressors ambient air is compressed to pressures on the order of 850.degree. F. at pressures which may be on the order of 400 psi and can flow at 450 feet per second as it passes through the compressor. Under these conditions commercial titanium alloys will burn uncontrollably if ignited. Ignition can occur by friction arising from the ingestion of foreign objects or as a result of mechanical failure which causes contact between moving and stationary titanium blade objects. Friction between titanium components is particularly troublesome. Such combustion is a great concern to gas turbine engine designers who have gone to great lengths to guard against rubbing between titanium components. However, it has to date been inherent physical characteristic of the titanium alloys used and an unavoidable potential consequence of using titanium in turbine compressor sections.
The assignee of the present invention has long standing expertise in the field of gas turbine engine technology and has devised a test for titanium alloy combustibility which comprises preparing a sample of 0.070 in sheet having a knife edge and placing this knife edge sample in an air stream flowing at 450 feet per second at a pressure of 400 psi and a temperature 850.degree. F. and attempting to ignite the sample using a 200 watt CO.sub.2 laser which impinges directly on the knife edge of the sample within the flowing gas stream. These test conditions are typical of those encountered in operating conditions in turbine engines. This test will be used hereinafter to define whether or not an alloy is burnable.
British Patent No. 1,175,683 to Imperial Metal Industries describes a titanium alloy which can contain 25-40% vanadium, 5-15% chromium up 10% aluminum balance titanium. Of 16 specific alloy compositions discussed in the patent only one contains more than 10% chromium and there is no appreciation shown in the patent for the effect of chromium on burnability of titanium alloys. U.S. Pat. No. 3,644,153 describes abrasion resistant materials formed by nitriding titanium alloy substrates. The substrate alloy may contain substantial amounts of vanadium and chromium. There is no disclosure in the patent of any mechanical properties in the substrate material per se nor of any nonburning properties in the substrate nor indeed of any substrate utility aside from as a material to be nitrided. U.S. Pat. No. 3,673,038 deals with a braze material for joining graphite and refractory materials. The braze material can consist of 10-45% vanadium, 5-20% chromium. Chromium is disclosed as providing flowability of the brazed material but no discussion presented concerning burnability.